The present invention relates generally to the detection of the position of a metal part or other element of electrically conductive material movable in a device or apparatus and, more particularly, to a means for determining the position of the disc of a check valve while the valve is in operation.
Check valves are widely employed in nuclear power plants, refineries and other chemical processing plants, pipelines and elsewhere to control fluid flow. The purpose of a check valve is to allow fluid flow in one direction and prevent flow in the reverse direction. The force of the fluid passing through the valve pushes the disc open in the forward flow direction, but if the fluid starts to reverse its direction of flow, because an upstream pump has been shut off or for whatever reason, the disc closes to stop the reverse flow, usually assisted by gravity and sometimes by springs. When the disc closes, it moves down to seal against the valve seat which is located on the upstream side of the valve, thus shutting off the flow. When flow reestablishes in the forward direction, the disc swings up out of the way to allow the flow to take place unimpeded. A backstop usually exists in the valve to restrain the travel of the disc in the opening direction, thereby establishing the disc's full open position.
It is important to know the position of the disc within the valve to verify full closure or full opening or to assess the degree of partial opening or to see the existence and magnitude of a disc fluttering condition or a backstop tapping condition, the latter two being detrimental to the long term life of the valve, as they cause damage or wear to internal hinge pins, disc studs and other valve internal parts. Because it is not possible to see the disc, it is necessary to ascertain its position and behavior by alternative means from the outside of the valve. Though it is possible to attach linkages or even magnets to the valve internals and monitor their motion from the outside of the valve, such devices require special attachment means, and, in the case of linkages, special sealing means, and this makes their use somewhat undesirable.
The present invention addresses the problem of externally monitoring the operation of such a valve or other enclosed, movable, electrically conductive element, while avoiding the use of special internal attachments or sealing means or any disassembly of the valve or device enclosing such element.